Reducing Refresh Power in Mobile Devices with Morphable ECC

Energy consumption is a primary consideration that determines the usability of emerging mobile computing devices such as smartphones. Refresh operations for main memory account for a significant fraction of the overall energy consumption, especially during idle periods, when processor can be switched off quickly; however, memory contents continue to get refreshed to avoid data loss. Given that mobile devices are idle most of the times, reducing refresh power in idle mode is critical to maximize the duration for which the device remains usable. The frequency of refresh operations in memory can be reduced significantly by using strong multi-bit error correction codes (ECC). Unfortunately, strong ECC codes incur high latency, which causes significant performance degradation (as high as 21%, and on average 10%). To obtain both low refresh power in idle periods and high performance in active periods, this paper proposes Morphable ECC (MECC). During idle periods, MECC keeps the memory protected with 6-bit ECC (ECC-6) and employs a refresh period of 1 second, instead of the typical refresh period of 64ms. During active operation, MECC reduces the refresh interval to 64ms, and converts memory from ECC-6 to weaker ECC (single-bit error correction) on a demand-basis, thus avoiding the high latency of ECC-6, except for the first access during the active mode. Our proposal reduces refresh operations during idle mode by 16x, memory power in idle mode by 2X, while retaining performance within 2% of a system that does not use any ECC.